Answer: It is ironic because in reality, he is Jack's brother
Explanation: Miss Prism identifies it, and Lady Bracknell reveals that Jack is Algernon's older brother, the son of Ernest John Moncrieff, who died years ago in India. Jack now truly is Ernest, and Algernon/Cecily, Jack/Gwendolen, and Chasuble/Prism fall into each others' arms as Jack realizes the importance of being earnest.